Image processing method and apparatus

Abstract

There are image processing method and apparatus for use in digital copying apparatuses, facsimile apparatuses, etc. which handle an image signal. This apparatus comprises an edge detector to detect the edge portion of the image signal, a smoothing circuit to smooth the image signal, an edge emphasizer to emphasize the edge of the image signal, and a mixer to mix the output of the smoothing circuit and the output of the edge emphasizer. A mixture ratio of the output of the smoothing circuit and the output of the edge emphasizer is varied in response to the output of the edge detector. The edge detection of the edge detector is performed due to the linear differentiation. With this apparatus, the moire in the case of a mesh original image is removed and the characters and line drawings can be reproduced with a high degree of fidelity.

What is claimed is: 
     
       1. An image processing apparatus comprising: detecting means for detecting a level of a predetermined spatial frequency, said predetermined spatial frequency corresponding to portions in an image signal which portions have a feature selected from the group consisting of character and line drawing, said detecting means outputting a detection signal corresponding to the level of said predetermined spatial frequency;   smoothing means for smoothing said image signal to produce a smoothed image signal; and   mixing means for mixing said smoothed image signal and said image signal,   wherein a mixture ratio of said mixing means is varied in response to said detection signal, and a component of said smoothed image signal becomes larger when said detection signal becomes smaller, and   wherein said mixture ratio is determined by an output of a continuous function using the level detected by said detecting means as a variable.   
     
     
       2. An image processing apparatus according to claim 1, wherein said detecting means consists of a linear differential circuit. 
     
     
       3. An image processing apparatus according to claim 2, wherein said detecting means obtains the sum of linear differentiations in two or more directions. 
     
     
       4. An image processing apparatus comprising: detecting means for detecting a level of a predetermined spatial frequency, said predetermined spatial frequency corresponding to portions in an image signal which portions have a feature selected from the group consisting of character and line drawing, said detecting means outputting a detection signal corresponding to the level of said predetermined spatial frequency;   smoothing means for smoothing said image signal to produce a smoothed image signal;   edge emphasizing means for edge-emphasizing said image signal to produce an edge-emphasized image signal; and   mixing means for mixing said smoothed image signal and said edge-emphasized image signal,   wherein a mixture ratio of said smoothed image signal and said edge-emphasized image signal is controlled in response to said detection signal, and a component of said smoothed image signal becomes larger when said detection signal becomes smaller, and   wherein said mixture ratio is determined by an output of a continuous function using the level detected by said detecting means as a variable.   
     
     
       5. An image processing apparatus according to claim 4, wherein said detecting means performs the detection by way of a linear differentiation. 
     
     
       6. An image processing apparatus according to claim 5, wherein said detecting means performs the detection based on a sum of linear differentiations in two or more directions. 
     
     
       7. An image processing apparatus according to claim 4, wherein said edge emphasizing means is a spatial filter having a high band emphasizing characteristic. 
     
     
       8. An image processing apparatus according to claim 4, wherein said edge-emphasized image signal is a sum of said image signal and an output obtained by multiplying an output of a spatial filter having a high-pass characteristic by a constant. 
     
     
       9. An image processing apparatus according to claim 4, wherein said edge-emphasized image signal is a sum of said image signal and an output obtained by multiplying an output of a spatial filter having a band-pass characteristic by a constant. 
     
     
       10. An image processing apparatus according to claim 4, wherein said edge emphasizing means has a spatial filter, and a peak frequency of said spatial filter is higher than said predetermined spatial frequency detected by said detecting means. 
     
     
       11. An image processing apparatus comprising: detecting means for detecting a level of a predetermined spatial frequency, said predetermined spatial frequency corresponding to portions in an image signal which portions have a feature selected from the group consisting of character and line drawing, said detecting means outputting a detection signal corresponding to the level of said predetermined spatial frequency;   a smoothing circuit for smoothing said image signal to produce a smoothed image signal;   mixing means for mixing said image signal and said smoothed image signal; and   converting means for non-linearly converting said detection signal to produce a converted detection signal,   wherein a mixture ratio of said mixing means is varied in response to said converted detection signal, and a component of said smoothed image signal becomes larger when said detection signal becomes smaller, and   wherein said mixture ratio is determined by an output of a continuous function using the level detected by said detecting means as a variable.   
     
     
       12. An image processing apparatus comprising: detecting means for detecting a level of a predetermined spatial frequency, said predetermined spatial frequency corresponding to portions in an image signal which portions have a feature selected from the group consisting of character and line drawing, said detecting means outputting a detection signal corresponding to the level of said predetermined spatial frequency;   a smoothing circuit for smoothing said image signal to produce a smoothed image signal;   an edge emphasizing circuit for emphasizing the edges of said image signal to produce an edge-emphasized image signal; and   converting means for non-linearly converting said detection signal to produce a converted detection signal,   wherein said smoothed image signal and said edge-emphasized image signal are mixed, in a mixture ratio controlled in response to said detection signal, and said smoothed image circuit becomes larger when said detection signal become smaller, and   wherein said mixture ratio is determined by an output of a continuous function using the level detected by said detecting means as a variable.   
     
     
       13. An image processing apparatus comprising: edge detecting means for detecting an edge portion of an image signal and outputting a detection signal;   a smoothing circuit for smoothing said image signal to produce a smoothed image signal;   an edge emphasizing circuit for emphasizing edges in said image signal to produce an edge-emphasized image signal; and   means for mixing said smoothed image signal and said edge-emphasized image signal to produce a mixed signal or selecting said smoothed image signal in response to the detection signal and then outputting said mixed signal or selected signal,   wherein said edge emphasizing circuit comprises a spatial filter having a peak frequency higher than a peak frequency of said edge detecting means.   
     
     
       14. An image processing apparatus comprising: edge detecting means for detecting an edge portion of an image signal and outputting a detection signal;   a smoothing circuit for smoothing said image signal to produce a smoothed image signal;   an edge emphasizing circuit for emphasizing edges in said image signal to produce an edge-emphasized image signal; and   means for mixing said smoothed image signal said edge-emphasized image signal to produce a mixed signal or selecting said smoothed image signal in response to the detection signal and then outputting said mixed signal or selected output,   wherein said edge detecting means comprises a spatial filter having a peak frequency lower than a primary harmonic frequency of a mesh image.   
     
     
       15. An image processing apparatus comprising: edge detecting means for detecting an edge portion of an image signal to produce a detection signal;   a smoothing circuit for smoothing said image signal to produce a smoothed image signal;   an edge emphasizing circuit for emphasizing edges in said image signal to produce an edge-emphasized image signal; and   means for mixing said smoothed image signal and said edge-emphasized image signal to produce a mixed signal or selecting said smoothed image signal or said edge-emphasized image signal in response to the detection signal and then outputting said mixed signal or selected output,   wherein said smoothing circuit comprises a spatial filter having a peak frequency characteristic near a primary harmonic frequency of a mesh image is sufficiently reduced.   
     
     
       16. An image processing method comprising the steps of: detecting a level of a predetermined spatial frequency corresponding to portions of character and line drawing in an input image signal, and outputting a detection signal corresponding to the level of the predetermined spatial frequency;   smoothing said image signal to produce a smoothed image signal;   mixing said image signal and the smoothed image signal to produce a mixed signal or selecting said image signal or the smoothed image signal and then outputting said mixed signal or the selected signal; and   controlling a ratio of said mixture and said selection in response to the detection signal output in said detecting step,   wherein a rate of a smoothed output in said smoothing step becomes larger when the detection signal becomes smaller, and   wherein said ratio is determined by an output of a continuous function using the level detected in said detecting step as a variable.   
     
     
       17. An image processing method comprising the steps of: detecting a level of a predetermined spatial frequency corresponding to portions of character and line drawing in an input image signal and outputting a detection signal corresponding to the level of the predetermined spatial frequency;   smoothing the image signal to produce a smoothed image signal;   emphasizing the edge of the image signal to produce an edge-emphasized image signal;   mixing the edge-emphasized image signal and the smoothed image signal to produce a mixed signal or selecting the edge-emphasized image signal or the smoothed image signal and then outputting the mixed signal or selected signal; and   controlling a ratio of said mixture and said selection in response to the detection signal output in said detecting step,   wherein an amount of a smoothed signal in said ratio becomes larger when the detection signal becomes smaller, and   wherein said ratio is determined by a continuous function using the level detected in said detecting step as a variable.   
     
     
       18. An image signal processing apparatus comprising: input means for inputting an image signal;   detection means for detecting a predetermined spatial frequency from the image signal to extract a character and line-drawing region, the predetermined spatial frequency corresponding to a character and a line drawing, said detection means producing a detection signal;   smooth means for smoothing the image signal to produce a smoothed image signal; and   edge emphasis means for edge-emphasizing the image signal to produce an edge-emphasized image signal,   wherein said edge emphasis means is enabled in response to the detection signal, and   wherein said smooth means is enabled with respect to lower and higher regions which regions are not detected by said detection means, said lower region representing a frequency region the frequency of which is lower than said predetermined spatial frequency, and said higher region representing a frequency region the frequency of which is higher than said predetermined spatial frequency.   
     
     
       19. An image signal processing apparatus according to claim 18, wherein a peak frequency of the detected spatial frequency of said detection means is lower than a peak frequency of said edge emphasis means. 
     
     
       20. An image signal processing apparatus according to claim 18, wherein said detection means consists of a linear differentiation circuit. 
     
     
       21. An image signal processing apparatus according to claim 20, wherein said detection means obtains the sum of linear differentiations in two or more directions. 
     
     
       22. An image signal processing apparatus according to claim 20, wherein a second peak of a frequency characteristic of said liner differentiation is set to be 2/3 of a first peak. 
     
     
       23. An image signal processing apparatus according to claim 18, further comprising combine means for combining said smoothed image signal and said edge-emphasized image signal with each other. 
     
     
       24. An image signal processing apparatus according to claim 23, wherein said combine means selects one of said smoothed image signal and said edge-emphasized image signal, responsive to the detection signal. 
     
     
       25. An image signal processing apparatus according to claim 23, wherein said combine means mixes said smoothed image signal and said edge-emphasized image signal to produce a mixed output, changing a mixing ratio of said smoothed image signal and said edge-emphasized image signal responsive to the detection signal. 
     
     
       26. An image signal processing apparatus according to claim 25, further comprising conversions means for non-linearly converting the output of said detection means. 
     
     
       27. An image signal processing apparatus according to claim 18, wherein said edge emphasis means has a spatial filter different from said detection means. 
     
     
       28. An image signal processing apparatus according to claim 18, wherein the detected peak frequency of said detection means is set to be lower than a primary harmonic frequency of mesh image. 
     
     
       29. An image signal processing apparatus according to claim 28, wherein the primary harmonic frequency is smoothed by said smooth means, to be eliminated. 
     
     
       30. An image signal processing apparatus according to claim 18, wherein said detection means, smooth means and edge emphasis means operate in parallel. 
     
     
       31. An image signal processing apparatus according to claim 18, wherein said detection means, smooth means and edge emphasis means operate by way of a convoluting method, and the sizes of nuclei necessary for the respective convolutions are equalized. 
     
     
       32. An image signal processing apparatus comprising: receiving means for receiving a signal representing an image;   detection means for detecting a spatial frequency in the image represented by the received signal;   smoothing means for smoothing the received image signal to produce a smoothed image signal;   edge-emphasis means for edge-emphasizing the received image signal to produce an edge-emphasized image signal; and   forming means responsive to said detection means for forming an output signal at least predominantly from the edge-emphasized image signal for regions in the image having a spatial frequency within a predetermined frequency range and at least predominantly from the smoothed image signal for regions in the image having a spatial frequency outside the predetermined frequency range.   
     
     
       33. An apparatus according to claim 32, wherein said detection means produces an output having a peak at a peak frequency within the predetermined frequency range, the peak frequency being lower than a peak frequency of said edge-emphasis means. 
     
     
       34. An image signal processing apparatus comprising: input means for inputting an image signal;   detector means for detecting a predetermined local frequency range of the image signal corresponding to a character and a line drawing range for extracting the character and line drawing range;   smoothing means for smoothing the image signal to produce a smoothed image signal; and   edge-emphasis means for edge-emphasizing the image signal to produce an edge-emphasized image signal,   wherein said edge-emphasis means is activated in the predetermined local frequency range in dependence on a detection result obtained by said detector means,   wherein said smoothing means is active as a low-pass filter in at least a frequency range which is other than the predetermined local frequency range in dependence on the detection range of said detector means, and   wherein said edge-emphasized image signal and said smoothed image signal are mixed in a mixer in a suitable ratio.   
     
     
       35. An apparatus according to claim 34, wherein a peak frequency of the detected local frequency of said detector means is lower than a peak frequency of said edge-emphasis means. 
     
     
       36. An image processing apparatus comprising: detecting means for detecting a level of feature of an input image signal;   first processing means for processing the image signal;   second processing means for processing the image signal, characteristics of said second processing means being different from characteristics of said first processing means; and   mixing means for mixing an output of said first processing means and an output of said second processing means at a predetermined ratio,   wherein said ratio is defined by an output of a continuous function using the level detected by said detecting means as a variable.   
     
     
       37. An apparatus according to claim 36, wherein the continuous function si a linear function. 
     
     
       38. An apparatus according to claim 36, wherein the feature is an edge component of the input image signal. 
     
     
       39. An apparatus according to claim 36, wherein said second processing means is delay means.